The female-determining gene F of the housefly, Musca domestica, acts maternally to regulate its own zygotic activity

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.

The genomes of a monogenic fly: views of primitive sex chromosomes

The production of male and female offspring is often determined by the presence of specific sex chromosomes which control sex-specific expression, and sex chromosomes evolve through reduced recombination and specialized gene content. Here we present the genomes of Chrysomya rufifacies, a monogenic blow fly (females produce female or male offspring, exclusively) by separately sequencing and assembling each type of female and the male. The genomes (> 25X coverage) do not appear to have any sex-linked Muller F elements (typical for many Diptera) and exhibit little differentiation between groups supporting the morphological assessments of C. rufifacies homomorphic chromosomes. Males in this species are associated with a unimodal coverage distribution while females exhibit bimodal coverage distributions, suggesting a potential difference in genomic architecture. The presence of the individual-sex draft genomes herein provides new clues regarding the origination and evolution of the diverse sex-determining mechanisms observed within Diptera. Additional genomic analysis of sex chromosomes and sex-determining genes of other blow flies will allow a refined evolutionary understanding of how flies with a typical X/Y heterogametic amphogeny (male and female offspring in similar ratios) sex determination systems evolved into one with a dominant factor that results in single sex progeny in a chromosomally monomorphic system.

The autoregulatory loop: A common mechanism of regulation of key sex determining genes in insects

Sex determination in most insects is structured as a gene cascade, wherein a primary signal is passed through a series of sex-determining genes, culminating in a downstream double-switch known as doublesex that decides the sexual fate of the embryo. From the literature available on sex determination cascades, it becomes apparent that sex determination mechanisms have evolved rapidly. The primary signal that provides the cue to determine the sex of the embryo varies remarkably, not only among taxa, but also within taxa. Furthermore, the upstream key gene in the cascade also varies between species and even among closely related species. The order Insecta alone provides examples of astoundingly complex diversity of upstream key genes in sex determination mechanisms. Besides, unlike key upstream genes, the downstream double-switch gene is alternatively spliced to form functional sex-specific isoforms. This sex-specific splicing is conserved across insect taxa. The genes involved in the sex determination cascade such as Sex-lethal (Sxl) in Drosophila melanogaster, transformer (tra) in many other dipterans, coleopterans and hymenopterans, Feminizer (fem) in Apis mellifera, and IGF-II mRNA-binding protein (Bmimp) in Bombyx mori are reported to be regulated by an autoregulatory positive feedback loop. In this review, by taking examples from various insects, we propose the hypothesis that autoregulatory loop mechanisms of sex determination might be a general strategy. We also discuss the possible reasons for the evolution of autoregulatory loops in sex determination cascades and their impact on binary developmental choices.

The evolving puzzle of autosomal versus Y-linked male determination in Musca domestica

Sex determination is one of the most rapidly evolving developmental pathways, but the factors responsible for this fast evolution are not well resolved. The house fly, Musca domestica, is an ideal model for studying sex determination because house fly sex determination is polygenic and varies considerably between populations. Male house flies possess a male-determining locus, the M factor, which can be located on the Y or X chromosome or any of the five autosomes. There can be a single M or multiple M factors present in an individual male, in heterozygous or homozygous condition. Males with multiple copies of M skew the sex ratio toward the production of males. Potentially in response to these male-biased sex ratios, an allele of the gene transformer, Md-tra(D), promotes female development in the presence of one or multiple M factors. There have been many studies to determine the linkage and frequency of these male determining factors and the frequency of Md-tra(D) chromosomes in populations from around the world. This review provides a summary of the information available to date regarding the patterns of distribution of autosomal, X-linked and Y-linked M factors, the relative frequencies of the linkage of M, the changes in frequencies found in field populations, and the fitness of males with autosomal M factors vs. Y-linked M. We evaluate this natural variation in the house fly sex determination pathway in light of models of the evolution of sex determination.

Conserved RNA cis-elements regulate alternative splicing of Lepidopteran doublesex

Doublesex (dsx) is a downstream key regulator in insect sex determination pathway. In Drosophila, alternative splicing of Dm-dsx gene is sex-specifically regulated by transformer (tra), in which the functional TRA promotes female-specific Dm-dsx. However, the sex determination pathway in Lepidoptera is not well understood; here we focused on alternative splicing of doublesex (dsx) in two agricultural pests, Asian corn borer (Ostrinia furnacalis) and cotton bollworm (Helicoverpa armigera), as well as the silkworm (Bombyx mori). More than a dozen new alternative splicing isoforms of dsx were found in the Lepidopteran females, which exist in all tested developmental stages and differentiated tissues. Alignment of mRNA and protein sequences of doublesex revealed high conservation of this gene in Lepidoptera. Strength analysis of splice sites revealed a weak 5' splice site at intron 3 in Lepidopteran dsx, which was experimentally confirmed. Furthermore, we identified highly conserved RNA sequences in the Lepidopteran dsx, including RNA elements I (14 nt), II (11 nt), III (26 nt), IV (17 nt), 3E-1 (8 nt) and 3E-2 (8 nt). The RNA elements III and IV were previously found in exon 4 of B. mori dsx and bound with Bm-PSI, which suppressed the inclusion of exons 3 & 4 into the male-specific Bm-dsx. Then we identified and analyzed the homologous genes of Bm-psi in the two Lepidopteran pests, which expressed at similar levels and exhibited a unique isoform in the males and females from each Lepidoptera. Importantly, mutagenesis of Bm-dsx mini-genes and their expression in BmN cell line demonstrated that three RNA elements are involved in the female-specific alternative splicing of Bm-dsx. Mutations in the RNA cis-elements 3E-1 and 3E-2 resulted in decreased inclusion of exon 3 into the female-specific dsx mRNA, suggesting that these two elements would be exonic splicing enhancers that facilitate the recognition of the weak 5' splice site at intron 3 of Lepidopteran dsx. We propose that the 5' splice sites at intron 3 are weak, resulting in multiple alternative splicing events in intron 3 of female Lepidoptera dsx. Activation of the 5' splice site requires regulatory cis-elements in exons 3 for female-specific splicing of Lepidoptera dsx.

Male death resulting from hybridization between subspecies of the gypsy moth, Lymantria dispar

We explored the origin of all-female broods resulting from male death in a Hokkaido population of Lymantria dispar through genetic crosses based on the earlier experiments done by Goldschmidt and by testing for the presence of endosymbionts that are known to cause male killing in some insect species. The mitochondrial DNA haplotypes of the all-female broods in Hokkaido were different from those of normal Hokkaido females and were the same as those widely distributed in Asia, including Tokyo (TK). Goldschmidt obtained all-female broods through backcrossing, that is, F1 females obtained by a cross between TK females (L. dispar japonica) and Hokkaido males (L. dispar praeterea) mated with Hokkaido males. He also obtained all-male broods by mating Hokkaido females with TK males. Goldschmidt inferred that female- and male-determining factors were weakest in the Hokkaido subspecies and stronger in the Honshu (TK) subspecies. According to his theory, the females of all-female broods mated with Honshu males should produce normal sex-ratio broods, whereas weaker Hokkaido sexes would be expected to disappear in F1 or F2 generations after crossing with the Honshu subspecies. We confirmed both of Goldschmidt's results: in the case of all-female broods mated with Honshu males, normal sex-ratio broods were produced, but we obtained only all-female broods in the Goldschmidt backcross and obtained an all-male brood in the F1 generation of a Hokkaido female crossed with a TK male. We found no endosymbionts in all-female broods by 4,'6-diamidino-2-phenylindole (DAPI) staining. Therefore, the all-female broods observed in L. dispar are caused by some incompatibilities between Honshu and Hokkaido subspecies.

About females and males: continuity and discontinuity in flies

Through the decades of relentless and dedicated studies in Drosophila melanogaster, the pathway that governs sexual development has been elucidated in great detail and has become a paradigm in understanding fundamental cell-fate decisions. However, recent phylogenetic studies show that the molecular strategy used in Drosophila deviates in some important aspects from those found in other dipteran flies and suggest that the Drosophila pathway is likely to be a derivative of a simpler and more common principle. In this essay, I will discuss the evolutionary plasticity of the sex-determining pathway based on studies in the common housefly, Musca domestica. Diversification appears to primarily arise from subtle differences in the regulation of the key switch gene transformer at the top of the pathway. On the basis of these findings I propose a new idea on how the Drosophila pathway may have evolved from a more archetypal system such as in M. domestica. In essence, the arrival of an X counting mechanism mediated by Sex-lethal to compensate for X linked gene dose differences set the stage for an intimate coupling of the two pathways. Its precedent recruitment to the dosage compensation pathway allowed for an intervention in the regulation of transformer where it gradually and eventually' completely substituted for a need of transformer autoregulation.

The transformer gene of Ceratitis capitata: a paradigm for a conserved epigenetic master regulator of sex determination in insects

The transformer gene in Ceratitis capitata (Cctra(ep)) is the founding member of a family of related SR genes that appear to act as the master epigenetic switch in sex determination in insects. A functional protein seems to be produced only in individuals with a female XX karyotype where it is required to maintain the productive mode of expression through a positive feedback loop and to direct female development by instructing the downstream target genes accordingly. When zygotic activation of this loop is prevented, male development follows. Recently, tra(ep) orthologues were isolated in more distantly related dipteran species including Musca domestica, Glossina morsitans and Lucilia cuprina and in the Hymenopterans Apis mellifera and Nasonia vitripennis. All of these tra(ep) orthologues seem to act as binary switches that govern all aspects of sexual development. Transient silencing leads to complete masculinization of individuals with a female karyotype. Reciprocally, in some systems it has been shown that transient expression of the functional TRA product is sufficient to transactivate the endogenous gene and implement female development in individuals with a male karyotype. Hence, a mechanism based on tra(ep) epigenetic autoregulation seems to represent a common and presumably ancestral single principle of sex determination in Insecta. The results of these studies will not only be important for understanding divergent evolution of basic developmental processes but also for designing new strategies to improve genetic sexing in different insect species of economical or medical importance.

The gene transformer-2 of Anastrepha fruit flies (Diptera, Tephritidae) and its evolution in insects

BACKGROUND

In the tephritids Ceratitis, Bactrocera and Anastrepha, the gene transformer provides the memory device for sex determination via its auto-regulation; only in females is functional Tra protein produced. To date, the isolation and characterisation of the gene transformer-2 in the tephritids has only been undertaken in Ceratitis, and it has been shown that its function is required for the female-specific splicing of doublesex and transformer pre-mRNA. It therefore participates in transformer auto-regulatory function. In this work, the characterisation of this gene in eleven tephritid species belonging to the less extensively analysed genus Anastrepha was undertaken in order to throw light on the evolution of transformer-2.

RESULTS

The gene transformer-2 produces a protein of 249 amino acids in both sexes, which shows the features of the SR protein family. No significant partially spliced mRNA isoform specific to the male germ line was detected, unlike in Drosophila. It is transcribed in both sexes during development and in adult life, in both the soma and germ line. The injection of Anastrepha transformer-2 dsRNA into Anastrepha embryos caused a change in the splicing pattern of the endogenous transformer and doublesex pre-mRNA of XX females from the female to the male mode. Consequently, these XX females were transformed into pseudomales. The comparison of the eleven Anastrepha Transformer-2 proteins among themselves, and with the Transformer-2 proteins of other insects, suggests the existence of negative selection acting at the protein level to maintain Transformer-2 structural features.

CONCLUSIONS

These results indicate that transformer-2 is required for sex determination in Anastrepha through its participation in the female-specific splicing of transformer and doublesex pre-mRNAs. It is therefore needed for the auto-regulation of the gene transformer. Thus, the transformer/transfomer-2 > doublesex elements at the bottom of the cascade, and their relationships, probably represent the ancestral state (which still exists in the Tephritidae, Calliphoridae and Muscidae lineages) of the extant cascade found in the Drosophilidae lineage (in which tra is just another component of the sex determination gene cascade regulated by Sex-lethal). In the phylogenetic lineage that gave rise to the drosophilids, evolution co-opted for Sex-lethal, modified it, and converted it into the key gene controlling sex determination.

Hormones and Sex-Specific Transcription Factors Jointly Control Yolk Protein Synthesis in Musca domestica

In the housefly Musca domestica, synthesis of yolk proteins (YPs) depends on the level of circulating ecdysteroid hormones. In female houseflies, the ecdysterone concentration in the hemolymph oscillates and, at high levels, is followed by expression of YP. In male houseflies, the ecdysterone titre is constantly low and no YP is produced. In some strains, which are mutant in key components of the sex-determining pathway, males express YP even though their ecdysterone titre is not significantly elevated. However, we find that these males express a substantial amount of the female variant of the Musca doublesex homologue, Md-dsx. The dsx gene is known to sex-specifically control transcription of yp genes in the fat body of Drosophila melanogaster. Our data suggest that Md-dsx also contributes to the regulation of YP expression in the housefly by modulating the responsiveness of YP-producing cells to hormonal stimuli.

Molecular characterization of the key switch F provides a basis for understanding the rapid divergence of the sex-determining pathway in the housefly

The housefly, Musca domestica, is an excellent model system to study the diversification of the pathway that specifies the sexual fate. A number of different mechanisms have been described in the housefly, which reflects in part the broad diversity of sex-determining strategies used in insects. In this study we present the molecular identification and characterization of F, which acts as the master switch in the housefly pathway. We provide evidence that F corresponds to the transformer ortholog in Musca (Mdtra), which, as a result of alternative processing, expresses functional products only in individuals committed to the female fate. We demonstrate that, once activated, a self-sustaining feedback loop will maintain the female-promoting functions of Mdtra. Absence of Mdtra transcripts in eggs of Arrhenogenic (Ag) mutant females suggests that maternally deployed Mdtra activity initiates this self-sustaining loop in the zygote. When an M factor is paternally transmitted to the zygote, the establishment of the loop is prevented at an early stage before cellularization and splicing of Mdtra shifts irreversibly to the male nonproductive mode. On the basis of the analysis of two mutant alleles we can explain the different sex-determining systems in the housefly largely as deviations at the level of Mdtra regulation. This plasticity in the housefly pathway may provide a suitable framework to understand the evolution of sex-determining mechanisms in other insect species. For instance, while sex determination in a close relative, the tsetse fly Glossina morsitans, differs at the level of the instructive signal, we find that its tra ortholog, Gmtra, is regulated in a mode similar to that of Mdtra.

Sexual development in Lucilia cuprina (Diptera, Calliphoridae) is controlled by the transformer gene

Insects use an amazing variety of genetic systems to control sexual development. A Y-linked male determining gene (M) controls sex in the Australian sheep blowfly Lucilia cuprina, an important pest insect. In this study, we isolated the L. cuprina transformer (Lctra) and transformer2 (Lctra2) genes, which are potential targets of M. The LCTRA and LCTRA2 proteins are significantly more similar to homologs from tephritid insects than Drosophila. The Lctra transcript is alternatively spliced such that only females make a full-length protein and the presence of six TRA/TRA2 binding sites in the female first intron suggest that Lctra splicing is autoregulated as in tephritids. LCTRA is essential for female development as RNAi knockdown of Lctra mRNA leads to the development of male genitalia in XX adults. Analysis of Lctra expression during development shows that early and midstage male and female embryos express the female form of Lctra and males express only the male form by the first instar larval stage. Our results suggest that an autoregulatory loop sustains female development and that expression of M inhibits Lctra autoregulation, switching its splicing to the male form. The conservation of tra function and regulation in a Calliphorid insect shows that this sex determination system is not confined to Tephritidae. Isolation of these genes is an important step toward the development of a strain of L. cuprina suitable for a genetic control program.

Changes in the frequency of YM versus IIIM in the housefly, Musca domestica L., under field and laboratory conditions

In the housefly, Musca domestica L., sex is usually determined by a dominant factor, M, located on the Y chromosome. However, there are 'autosomal male' (AM) populations in which the M factor is located on one or more of the five autosomes (I-V) or on X. We examined changes in the frequency of AM and YM males in North Carolina populations of houseflies after 4 years in the laboratory (NC Lab 02:06) and after 4 or 5 years in the field (NC 2006 and NC 2007). In 2002, 77.7% of the male houseflies were III/III;XYM, 20% were IIIM/III;XX, and 2.3% were IIIM/III;XYM. After 4 years in the laboratory, IIIM/III males disappeared and 17.4% of the males were XMYM. Conversely, 4 years later, the field population was relatively unchanged from 2002. Thus, there was a strong selection against IIIM/III males in the laboratory, but not in the field. Field-collected flies from 2007 indicated a slight increase in the frequency of XYM males and a slight decrease in the frequency of IIIM/III males (relative to 2002 and 2006), suggesting that the relative frequency of XYM and IIIM/III can vary slightly over time in field populations. The detection of XMYM males in 2007 offered the opportunity to evaluate the frequency of the female-determining FD factor, which was found to be present in both the laboratory and field populations, but frequencies varied greatly. The present study represents the first report of FD in houseflies from North America. The significance of these results, relative to observed clines in AM versus YM males, is discussed.

Sex determination in the haplodiploid wasp Nasonia vitripennis (Hymenoptera: Chalcidoidea): a critical consideration of models and evidence

Sex determining mechanisms are highly diverse. Like all Hymenoptera, the parasitic wasp Nasonia vitripennis reproduces by haplodiploidy: males are haploid and females are diploid. Sex in Nasonia is not determined by complementary alleles at sex loci. Evidence for several alternative models is considered. Recent studies on a polyploid and a gynandromorphic mutant strain point to a maternal product that is balanced against the number of chromosomal complements in the zygote and a parent-specific (imprinting) effect. Research is now focused on the molecular details of sex determination in Nasonia.

Sex ratio selection and multi-factorial sex determination in the housefly: a dynamic model

Sex determining (SD) mechanisms are highly variable between different taxonomic groups and appear to change relatively quickly during evolution. Sex ratio selection could be a dominant force causing such changes. We investigate theoretically the effect of sex ratio selection on the dynamics of a multi-factorial SD system. The system considered resembles the naturally occurring three-locus system of the housefly, which allows for male heterogamety, female heterogamety and a variety of other mechanisms. Sex ratio selection is modelled by assuming cost differences in the production of sons and daughters, a scenario leading to a strong sex ratio bias in the absence of constraints imposed by the mechanism of sex determination. We show that, despite of the presumed flexibility of the SD system considered, equilibrium sex ratios never deviate strongly from 1 : 1. Even if daughters are very costly, a male-biased sex ratio can never evolve. If sons are more costly, sex ratio can be slightly female biased but even in case of large cost differences the bias is very small (<10% from 1 : 1). Sex ratio selection can lead to a shift in the SD mechanism, but cannot be the sole cause of complete switches from one SD system to another. In fact, more than one locus remains polymorphic at equilibrium. We discuss our results in the context of evolution of the variable SD mechanism found in natural housefly populations.

The transformer2 gene in Musca domestica is required for selecting and maintaining the female pathway of development

We present the isolation and functional analysis of a transformer2 homologue Mdtra2 in the housefly Musca domestica. Compromising the activity of this gene by injecting dsRNA into embryos causes complete sex reversal of genotypically female individuals into fertile males, revealing an essential function of Mdtra2 in female development of the housefly. Mdtra2 is required for female-specific splicing of Musca doublesex (Mddsx) which structurally and functionally corresponds to Drosophila dsx, the bottom-most regulator in the sex-determining pathway. Since Mdtra2 is expressed in males and females, we propose that Mdtra2 serves as an essential co-factor of F, the key sex-determining switch upstream of Mddsx. We also provide evidence that Mdtra2 acts upstream as a positive regulator of F supporting genetic data which suggest that F relies on an autocatalytic activity to select and maintain the female path of development. We further show that repression of male courtship behavior by F requires Mdtra2. This function of F and Mdtra2 appears not to be mediated by Mddsx, suggesting that bifurcation of the pathway at this level is a conserved feature in the genetic architecture of Musca and Drosophila.

Sex determination in Drosophila melanogaster and Musca domestica converges at the level of the terminal regulator doublesex

Sex-determining cascades are supposed to have evolved in a retrograde manner from bottom to top. Wilkins' 1995 hypothesis finds support from our comparative studies in Drosophila melanogaster and Musca domestica, two dipteran species that separated some 120 million years ago. The sex-determining cascades in these flies differ at the level of the primary sex-determining signal and their targets, Sxl in Drosophila and F in Musca. Here we present evidence that they converge at the level of the terminal regulator, doublesex ( dsx), which conveys the selected sexual fate to the differentiation genes. The dsx homologue in Musca, Md-dsx, encodes male-specific (MdDSX(M)) and female-specific (MdDSX(F)) protein variants which correspond in structure to those in Drosophila. Sex-specific regulation of Md-dsx is controlled by the switch gene F via a splicing mechanism that is similar but in some relevant aspects different from that in Drosophila. MdDSX(F) expression can activate the vitellogenin genes in Drosophila and Musca males, and MdDSX(M) expression in Drosophila females can cause male-like pigmentation of posterior tergites, suggesting that these Musca dsx variants are conserved not only in structure but also in function. Furthermore, downregulation of Md-dsx activity in Musca by injecting dsRNA into embryos leads to intersexual differentiation of the gonads. These results strongly support a role of Md-dsx as the final regulatory gene in the sex-determining hierarchy of the housefly.

Genetic control of germline sexual dimorphism in Drosophila

Females produce eggs and males produce sperm. Work in Drosophila is helping to elucidate how this sex-specific germline differentiation is genetically encoded. While important details remain somewhat controversial, it is clear that signals generated by somatic cells, probably in the embryonic gonads, are required as extrinsic factors for germline sex determination. It is equally clear that the sex chromosome karyotype of the germ cell is an intrinsic factor for germline sex determination. There is also extensive somatic signaling required for differentiation of germline cells in the adult gonads. Mismatched germline and somatic line sexual identities place germ cells in an inappropriate signaling milieu, which results in either failed maintenance of germline stems cells when female germ cells are in a male soma or overproliferation of germline cells when male germ cells are in a female soma. The well-studied somatic sex determination genes including transformer, transformer-2, and doublesex are clearly involved in the nonautonomous signaling from somatic cells, while the autonomous functions of genes including ovo, ovarian tumor, and Sex-lethal are involved in the germline. The integration of these two pathways is not yet clear.

Thetransformergene inCeratitis capitataprovides a genetic basis for selecting and remembering the sexual fate

The medfly Ceratitis capitata contains a gene (Cctra) with structural and functional homology to the Drosophila melanogaster sex-determining gene transformer (tra). Similar to tra in Drosophila, Cctra is regulated by alternative splicing such that only females can encode a full-length protein. In contrast to Drosophila, however, where tra is a subordinate target of Sex-lethal (Sxl), Cctra seems to initiate an autoregulatory mechanism in XX embryos that provides continuous tra female-specific function and act as a cellular memory maintaining the female pathway. Indeed, a transient interference with Cctra expression in XX embryos by RNAi treatment can cause complete sexual transformation of both germline and soma in adult flies, resulting in a fertile male XX phenotype. The male pathway seems to result when Cctra autoregulation is prevented and instead splice variants with truncated open reading frames are produced. We propose that this repression is achieved by the Y-linked male-determining factor (M).

Maternal-offspring conflict leads to the evolution of dominant zygotic sex determination

Sex determination in many species involves interactions among maternally expressed genes (eg, mRNA's and proteins placed into the egg) and zygotically expressed genes. Recent studies have proposed that conflicting selective pressures can occur between maternally and zygotically expressed sex determining loci and that these may play a role in shaping the evolution of sex determining systems. Here we show that such genetic conflict occurs under very general circumstances. Whenever sex ratio among progeny in a family affects the fitness of either progeny in that family or maternal fitness, then maternal-zygotic genetic conflict occurs. Furthermore, we show that this conflict typically results in a "positive feedback loop" that leads to the evolution of a dominant zygotic sex determining locus. When males more negatively effect fitness within the family, a male heterogametic (XY male) sex determining system evolves, whereas when females more negatively effect fitness in the family, a female heterogametic (ZW female) system evolves. Individuals with the dominant sex allele are one sex, and the opposite sex is determined by maternally-expressed genes in individuals without the dominant sex allele. Results therefore suggest that maternal-zygotic conflict could play a role in the early evolution of chromosomal sex determining systems. Predictions are made concerning the patterns of expression of maternal and zygotic sex determining genes expected to result from conflict over sex determination.

Genetic transformation of the housefly Musca domestica with the lepidopteran derived transposon piggyBac

The piggyBac transposable element was successfully used for stable genetic transformation of the housefly Musca domestica. The construct contains the EGFP marker under the control of Pax-6 binding sites, which can drive eye-specific expression in insect species as distantly related as Drosophila melanogaster and Tribolium castaneum [Berghammer, A.J., Klingler, M. and Wimmer, E.A. (1999) Nature 402: 370-371]. We obtained seven independent integration events among 41 fertile G0 Musca flies. Most of the transformed lines contained two or more chromosomal insertions of the EGFP marker which were stably inherited over more than 15 generations. piggyBac-mediated transposition was verified by identifying the characteristic TTAA duplication at the insertion sites. This first report of stable transmission of a genetic marker in Musca confirms the use of this vector-marker system for effective gene transfer in a broad range of insect species.

The male-determining activity on the Y chromosome of the housefly (Musca domestica L.) consists of separable elements

In the common housefly, the presence or absence of a male-determining factor, M, is responsible for sex determination. In different strains, M has been found on the Y, on the X, or on any of the five autosomes. By analyzing a Y-autosomal translocation and a ring-shaped, truncated Y chromosome, we could show that M on the Y consists of at least two regions with M activity: One of them can be assigned to the short arm of the Y chromosome (MYS), which is largely C-banding negative, the other region lies on the C-banding positive long arm of the Y, including the centromeric part (MYL). Each region alone behaves as a hypomorphic M factor, causing many carriers to develop as intersexes of the mosaic type instead of as males. When introduced into the female germ line by transplantation of progenitor germ cells (pole cells), the MYS shows an almost complete maternal effect that predetermines 96% of the genotypic female (NoM) animals to develop as males. In contrast, the MYL has largely lost its maternal effect, and most of the NoM animals develop as females. Increasing the amount of product made by either of the two hypomorphic M factors (by combining the MYS and MYL or two MYS) leads to complete male development in almost every case. We thus assume that the Y chromosome carries at least two copies of M, and that these are functionally equivalent.